Investigate how surface colour and finish affect thermal radiation. Compare otherwise identical containers with different outer surfaces, such as matt black, matt white, shiny metallic and dull metallic.Original labelled apparatus schematic; not to scale. Follow the stated measurements and safety instructions.
Use identical beakers or tubes with the same exposed area, lids, thermometers or temperature probes, stopwatch and equal amounts of hot water. This cooling method tests the net energy loss under controlled conditions.
Hot water can scald. Use a stable bench, suitable heat protection and teacher-approved temperatures; avoid carrying overfilled hot containers.
Thermal radiation is electromagnetic energy transfer, mainly infrared at these temperatures. Conduction and convection also occur, so the design must reduce or control them.
Cooling method and controls
Fill each container with the same mass or volume of water at the same initial temperature. Start monitoring promptly; filling order must not produce different unrecorded cooling intervals.
Fit equivalent lids to reduce evaporation and convection from the water surface. Place temperature probes at the same depth without touching the container wall or base.
Keep room conditions, container spacing, contact with the bench and exposure to draughts the same. Use identical containers and equal water amounts. This helps show the effect of surface finish, rather than differences in material, area or how much energy a container needs to warm up (thermal capacity).
Record each container’s temperature at regular intervals using a logger or coordinated timing. Stir gently in the same way if required, so the probe measures the temperature of the water as a whole rather than one warmer or cooler spot.
Calculate temperature fall over a common time or compare cooling-curve gradients at the same temperature. Use repeat trials and average comparable measures.
A larger temperature fall over the same interval suggests a greater net cooling rate. Matt black is generally a better infrared emitter than a shiny metallic surface; shiny surfaces are poor emitters and good reflectors.
All surfaces radiate and absorb simultaneously. The net loss depends on their temperature relative to surroundings; do not say a shiny surface emits no radiation.
To test absorption separately, expose comparable surfaces to the same radiant source and measure heating under controlled conditions. Cooling observations alone do not directly measure absorption of a lamp's energy.
Precision and evaluation
Plot temperature against time with labelled curves. A steeper negative gradient means faster cooling; temperature fall/time is an average rate, not necessarily a constant instantaneous rate.
Compare gradients at a common temperature because cooling rate changes with the water-to-room temperature difference. Identical starting temperatures make early comparisons fairer.
Use the same calibrated probe or cross-check probes; an instrument offset could otherwise be mistaken for a surface effect. Repeated readings do not remove that offset.
An insulated base and lids can reduce competing heat-transfer routes, but explain their purpose and keep the exposed side surfaces available to radiate.
Apply surface coatings with similar thickness. Different container masses, insulation or wall materials could change cooling as well as how well the surface emits radiation (its emissivity).
Quote measured cooling differences and spread. If repeat ranges overlap substantially, report the weak evidence rather than forcing an ordering between very similar surfaces.
Exam skills: planning, precision and evaluation
State what you change (the independent variable), what you measure (the dependent variable) and what you keep the same (control variables). Explain how you keep each control variable constant, rather than just saying “make it fair”.
Accuracy means how close a result is to the true value. Precision means how close repeated measurements are to each other. Resolution is the smallest change an instrument can show. More digits on a display do not automatically mean a more accurate result.
Repeat measurements for each condition, calculate a mean and describe how spread out the results are. This helps assess and reduce the effect of random errors. Repeating cannot fix an error that pushes results consistently in one direction (a systematic error), such as a temperature-probe offset mistaken for a surface effect.
Repeatability means getting similar results when the same person repeats the same method with the same equipment. Reproducibility means getting similar results when someone else, or different suitable equipment, repeats the experiment. Results can be consistent but still inaccurate.
Check that instruments read zero correctly and are calibrated where needed. Read scales at eye level: looking from an angle can give a wrong reading (parallax error). Choose suitable ranges, measurement intervals and scale divisions (resolution).
Write down the original readings straight away in a table, with units in the headings. Use decimal places that match the instrument’s resolution. Keep the original data and round only when needed. Do not discard a result just because it differs from your prediction.
An anomalous result does not fit the pattern of the other results. Repeat that measurement and check the method. Only leave it out of a mean if you have a clear reason; state which result you excluded and why.
For continuous variables, plot the independent variable on the horizontal axis and the dependent variable vertically. Use sensible scales, units and a best-fit line or curve; do not automatically join every point or force the graph through zero.
Find the gradient of a straight best-fit line using a large triangle: vertical change ÷ horizontal change. For a curve, draw a tangent to estimate the gradient at one point. Explain what the gradient shows in this experiment, include its units and use measured values to support your conclusion.
Uncertainty describes the possible range around a measurement. For one reading on a scale, half the smallest division is a useful classroom estimate unless the question says otherwise. If you subtract two readings, both have uncertainty. Percentage uncertainty = absolute uncertainty ÷ measured value × 100. Follow the method specified in the question.
Use results as evidence and then explain what they mean. A pattern linking variables (a correlation) does not prove that one causes the other. If the ranges of repeat results overlap, a claimed difference may be less convincing. Keep conclusions within the range tested and suggest an improvement that tackles a specific error.
Mr Kidman's Classroom